The invention relates to channelizing a digital spectrum into frequency bands when the input Analog to Digital Converter (ADC) sampling rate is constrained to a given value and, a specific but arbitrary, output sampling rate is desired.
Frequency Division Multiple Access (FDMA) provides access for multiple users to a single channel by subdividing the channel into multiple sub-channels. Each user is allocated one or a subset of all the channels and the user""s signal is used to modulate a carrier of the allocated sub-channel. A single-channel receiver may be used to demodulate a single assigned channel, or a multichannel receiver may demodulate a selected range of channels.
In an analog system, each receiver has a local oscillator (LO) tuned to the frequency of the channel to be demodulated. The LO signal is multiplied by the received signal to generate an intermediate frequency (IF) signal that is applied to a band-pass filter (BPF). The BPF has a passband selected to filter out all signals except the channel to be received. The output signal from the BPF can be digitized and processed subsequently, if desired.
Referring to FIG. 1, a typical receiver 100 is shown with a local oscillator (LO) 105 of frequency xcfx89. A signal mixer 110 mixes the amplified incoming FDMA signal 106 with LO 105 output to produce a signal with a fixed intermediate frequency (IF) 107. An analog BPF 111 selects the desired channel and an analog-to-digital converter (ADC) 115 converts the resulting output to digital form by sampling the analog signal at an appropriate frequency. Generally, the sampling frequency is at least twice the channel bandwidth to satisfy the Nyquist requirement.
The sampled digital data, x(n), is bandshifted digitally by digital detector 116 by multiplying with a phasor exe2x88x92j2xcfx80(kn/N) denoted by WNkn, where k denotes the channel selected by the receiver. When there are multiple channels contained in the received signal then one such receiver is needed for each of the channels. The resulting signal is low-pass filtered using a digital low pass filter 117. All of the processes following digitization of the band-passed IF signal can be performed with a digital computer or specialized circuits well known in the art.
Referring to FIG. 2, in a conventional polyphase approach to digital channelization, M channels of bandwidth B are received simultaneously in a FDMA signal 106. (Background on polyphase DSP can be found in R. E. Crochiere and L. E. Rabiner, Multirate Digital Signal Processing, Prentice Hall, Englewood Cliffs, N.J., 1983; and N. J. Fiege, Multirate Digital Signal Processing, John Wiley and Sons, 1994; the entire contents of each of which is incorporated herein by reference.) After M channels, each of bandwidth B, are passed through an analog BPF having a passband of at least MB (not shown) they are output to an A/D converter 140 which samples at some rate that is at least equal to the Nyquist rate (2MB) for a signal of bandwidth MB. In FIG. 2, the data is sampled at 2NB where N is greater than or equal to M. The digital output x(n) is applied to a 2N pole commutator 141 which distributes the input data x(n) to 2N filters 150. Each filter 150 is updated once every 2N points. Filters 150 perform the channel extraction function. The time series output of filters 150 is applied to respective inputs of an FFT processor 160 which processes the data once every 2N points to produce 2N complex outputs of which M outputs are chosen, each representing the bandshifted subchannel signal at B Hz, the update rate of FFT processor 160. Only M outputs of FFT processor 160 are required since the sample rate 2NB, as mentioned, can be higher than the minimum required sample rate 2MB.
The prior art approach cannot effectively accommodate signal processing subsequent to FFT processing that requires a sample rate that is independent of the sample rate of the A/D converter.